Electrolytic cavity sinking apparatus



March 14, 1967 J, L. BENDER ETAL 3,309,303

ELECTROLYTIC CAVITY SINKING APPARATUS Original Filed June 21,- 1960fm/enionq:

d W fwFd w/ United States Patent Ofiice 3,309,303 Patented Mar. 14, 1967ELECTROLYTIC CAVITY SINKING APPARATUS Joseph L. Bender, Wheeling, andLynn A. Williams,

Winnetka, Ill., assignors to Anocut Engineering Company, Elk GroveVillage, 11]., a corporation of Illinois Original application June 21,1960, Ser. No. 37,766, now

Patent No. 3,214,360, dated Oct. 26, 1965. Divided and this applicationJune 15, 1965, Ser. No. 464,045

7 Claims. (Cl. 204-224) This application is a division of copendingapplication Ser. No. 37,766, filed June 21, 1960, entitled, ElectrolyticCavity Sinking Apparatus, now issued into Patent No. 3,214,360, datedOct. 26, 1965.

The present invention relates generally to electrolytic cavity sinkingor shaping apparatus of the type disclosed in Lynn A. Williams PatentNo. 3,058,895, dated Oct. 16, 1962, for Electrolytic Shaping, and in thecopending aplication of Lynn A. Williams and James E. Davis, Ser. No.436,383, filed Dec. 23, 1964, for Control and Operating System forElectrolytic Hole Sinking.

The present invention is concerned with controlling the action of theelectrode and the electrolyte so as substantially to eliminate or reducetroublesome lateral vibration of electrodes used in both cavity sinkingand workpiece shaping operations, and to control the breakthrough of theelectrode at the remote side of the workpiece to produce a true hole oropening without irregularities and without excessive sparking betweenthe workpiece and the electrode.

In general, apparatus of the type disclosed in the foregoing patent andapplication includes a fixture for securely mounting the workpiece, ahollow electrode having a working tip of electrically conductivematerial, a ram head mounting the electrode for movement toward theworkpiece, an electric power supply connected to the electrode and tothe workpiece so as to make the workpiece anodic and the electrodecathodic and capable of delivering a low voltage to 15 volts), highdensity (500 to 8000 amperes per square inch) direct current, and asource of electrolyte capable of delivering electrolyte at a highvelocity in the work gap between the electrode and the workpiece and ata pressure of about 150 to 200 pounds per square inch.

It has been found from practical experience that one of the difiicultiesin obtaining the maximum rate of penetration of the electrode into theworkpiece is that the electrode tends to vibrate laterally, therebycausing short-circuiting against one of the side surfaces, either on theinternal bore of the electrode or externally, or both. now has beendiscovered that one of the things which causes or effects the lateralvibration is the tendency to store energy within the column of liquidinside the electrode. The cause of the oscillation which usually occursat a frequency within the sonic range appears to be the fact that theelectrolyte is being delivered in the work gap at an exceedingly highvelocity and under a very high pressure in the range, as previouslystated, of 150 to 200 pounds per square inch. When it is considered thatthe work gap between the electrode and the surface of the workpiecebeing acted upon is in the order of .002", it is not surprising thatthere is a feed-back oscillation in the column of liquid in theelectrodes passageway. It is a principal object of the present inventionto provide means associated with the electrode and the feed ofelectrolyte to dampen or break up this oscillation and thereby eliminatethe lateral vibration of the electrode within the hole being formed.

Another problem which has been encountered is in connection with throughbores or holes being formed in a workpiece. When the electrolyte is fedto the workpiece through the bore of the electrode, then as theelectrode approaches the remote or exit side of the workpiece, it willusually break through in some one area before it breaks through all theway around. This is due to the almost unavoidable irregularities in thesurface of the workpiece. When this occurs, the electrolyte then findsits way out through the broken-through opening at the exit side, insteadof turning back over the working tip of the electrode to return alongits axis to the entry surface of the workpiece. This is detected byobserving a jet of electrolyte squirting through an opening on the exitsurface of the workpiece. In Patent No. 3,058,895 arrangements aredisclosed to prevent this. One of them is to provide a dummy piecefastened to the exit surface of the workpiece by some adhesive such aswater glass. In another form, a backup material of soft rubber is used.It is obvious that these arrangements involve some inconvenience, and insome cases they are very diflicult to use because the exit surface ofthe workpiece may lead into a more or less closed chamber which is notreadily accessible. Unless some form of backup device is used, andunless the electrode is advanced so that its working tip has passedcompletely through the workpiece and into the backup device, there willbe in the cavity adjacent the exit surfaces a sharp, inturned lip.

It is, therefore, another primary object of the present invention toprovide a new arrangement for feeding the electrolyte and advancing theelectrode which will overcome the difficulties encountered in the breakthrough of the electrode from the exit side of the workpiece.

It should be noted that to a large extent the arrange ment forovercoming the lateral vibration also improves the exitingcharacteristics of the electrode feed.

Another object is to provide new and improved electrolytic cavitysinking apparatus wherein the electrolyte is fed along the outer surfaceof the electrode to the work gap to exit through the bore of theelectrode.

Another object is to provide a new and improved electrolytic cavitysinking apparatus wherein the electrolyte is fed in such fashion thatany column of electrolyte within the electrode is inhibited againstlongitudinal oscil lation which will provide lateral vibration of theelectrode.

Other objects and advantages will become apparent from the followingdescription taken in conjunction with the accompanying drawings,wherein:

FIG. 1 is a schematic view of a portion of an electrolytic cavitysinking apparatus illustrating one form of the present invention;

FIG. 2 is a schematic view of an arrangement for backing up the cavityin the workpiece to assist in making clean In the drawings, there hasbeen shown only so much of an electrolytic cavity sinking machine orapparatus as is necessary to an understanding of the present invention.Such apparatus is more completely disclosed in the aforementioned PatentNo. 3,058,895.

Referring now to FIG. 1, the apparatus includes an electrode structure,indicated generally by the reference character 10, which is fastened bya flange 12 to a ram plate 14. The ram plate is mounted on the forwardend of a ram sturcture,.and is insulated therefrom by an insulatingblock. The ram structure includes a movable member powered from apositively driven screw (not shown) which will advance the electrode 10at a constant speed into an electrically conductive andelectrochemically erodable workpiece W. The machine includes an electricpower supply shown diagrammatically at 15, the output of which isconnected by a cable or conductor 16 to the ram plate 14 and electrode10, and a conductor 18 to the work through the table and work holderwhich normally supports the workpiece W in place. The power supply iscapable of delivering, as previously mentioned herein, a low voltage to15 volts), high density (500 to 8000 amperes per square inch) directcurrent in a sense to make the electrode negative and the workpiece Wpositive.

The electrode structure 10 includes the electrode proper at 20 having aworking tip 22 of slightly greater diameter than the shank or body ofthe electrode 20. The shank or body is coated, at least on its exteriorsurface, with an insulatnig material 24, which may be a vitreous enamelor an epoxy resin. diameter, the interior also will be provided with acoating of insulation similar to the insulaion 24.

The electrode 20 is made integral with or commonly mounted in anenlarged tubular, piston-like portion 26 which serves as the electrodemount, and which in turn is secured to the flange 12. Mounted on theelectrode mount 26 is a bushing 28 Which may be made of a plasticmaterial impervious to chemical deterioration or damage by heat.Experimentally, an acrylic resin has been used and found to besatisfactory, but for a permanent insulation a more durable material,such as Teflon resin, is preferred. The feed bushing 28 is mounted sothat it slides easily over the electrode mount 26 and has a clearancetherewith in the order of .002". To prevent excessive leakage ofelectrolyte a number of squared-bottomed, annular grooves 30 are cutinto the external surface of the electrode mount, and they provide asuflicient seal to prevent excesisve leakage of electrolyte between thefeed bushing 28 and the mount 26. The feed bushing is made in such a Waythat at the inner end of its cylindrical bore 32 there is provided anannular opening 34 into which the electrolyte is fed under pressurethrough a supply line 36 and a fitting 38. The supply line 36 isconnected to a source of electrolyte under pressure which includes asupply tank or reservoir 35 and a pump 37 capable of deliveringelectrolyte at a pressure of 150 to 200 pounds per square inch.

The bushing 28 is formed with a bore 40 at its exit end which is justlarge enough to permit being slipped over the working tip 22 of theelectrode 20 so that there is a clearance of several thousandths of aninch between the bore 40 of the bushing and the shank of the electrode20, thereby permitting the easy and adequate flow of electrolyte alongthe side wall of the electrode and down over the working tip 22. At itsexit end the guide bushing 28 is relieved to provide a sealing shoulder42 of reduced cross sectional area to permit sealing against the entrysurface of the workpiece W. Within the confines of the shoulder there isa recess 44 which is provided to accommodate the tip 22 of the electrodeto prevent its coming into contact with the entry surface of theworkpiece W when the electrode is first advanced toward the workpieceand the initial erosion of the workpiece is being effected.

The feed bushing 28 is more or less permanently mounted on the electrodemount 26, and the entire assem- If the electrode is of any substantialbly is then brought into close proximity with the workpiece W with thebushing extended so that its sealing shoulder 42 touches the workpiecewhile the working tip 22 of the electrode is slightly recessed withinthe bushing at 44. As electrolyte is fed through the flexible tube 36and fitting 38 into the chamber 34, the effect is to create ahydrostatic force on the surface 46 of the chamber 34, thus urging thebushing into sealing engagement with the entry surface of the workpiece.Since the area of the frontal surface 46 is substantially in excess ofthe effective area of the reduced sealing shoulder 42, the effect of anyincrease in work pressure from the ram structure is increased sealingpressure, thereby completely eliminating the need for any mechanicalclamp or the like.

The electrolyte finds its way along the exterior surface of theelectrode through the space in the bore 40 between the electrode and thesurface of the bore, over the edge of the working tip 22, and backthrough the bore 48 in the electrode 20. The exit path may be valvedthrough a needle valve and gauge if it is desired to observe and adjustthe back pressure. In the case of electrodes Which are intended to beuesd many times in repetitive production operations, a simple exit holemay be provided in the electrode at its back end. In this case the borethrough the electrode mount is closed, as shown by the flange 12. If itis desired to adjust the back pressure, such may be done by an outletbushing, needle valve, and conduit leading to a drain. The drain, showndiagrammatically in FIG. 1, comprises a pan 49 in the bottom of the workarea and a conduit 51 leading to the reservoir 35. By arranging the exithole 50 so that it opens up wardly and is visible, it is possible tohave an excellent visual indication of the rate of electrolyte flow.This can be used for adjusting the in-feed rate of the electrode 20. Thein-feed rate is increased up to the point where the reduction in theheight of the fountain stream through the exit hole 50 indicates thatthe free flow of electrolyte is being impeded by too close a gap betweenthe working tip 32 and the workpiece. This follows the teachings ofPatent No. 3,958,895 in that the fountain column of liquid constitutes atype of flow meter. The amount of back pressure in the work gap isdetermined by the size of the exit hole 50, and this usually will beadjusted so that with an in-fed pressure in the order of 50 to 200p.s.i., the back pressure at the exit hole 50 will range fromapproximately one foot head of electrolyte solution to a pressure of theorder of a half, or even more than that, of the inlet pressure.

It will be appreciated that this arrangement maintains a solid column ofliquid all the way around the perimeter of the working tip 22. Inconsequence of this, the amount of side action is increased over thatwhich occurs when the electrolyte is applied to the bore of theelectrode. The result is to increase the overcut; that is, the excess ofthe size of the hole in the work with respect to the size of theelectrode :working tip. Accordingly, either the electrode must beadvanced more rapidly, the supply voltage of direct current must belowered, the width (in the direc tion of advance) of the working tipmust be narrowed, or some other means, such as an increase in allowancefor the amount of overcut, must be taken.

When the electrolyte is fed around the outside of the electrode, theproblem of breakthrough at the exit side is minimized, at least in thoseinstances where the plane of the working tip and the plane of the exitsurface are generally parallel. -What happens is that, with the liquid,during the operation, passing completely around the outside of theelectrode and back through its bore with the electrode approaching theexit surface, a breakthrough may be achieved at one point prior toanother, but the liquid will continue topass over the entire active areaof the working tip on its way to either of the exit paths; that is, theexit path through the bore 48 of the electrode, or through the newlycreated opening in the exit surface. In this way the electrode can beadvanced so that the working tip passes entirely through the workpiece,eliminating any inturned lip.

It has a so been found that with this arrangement the electrode 20 isless subject to lateral vibration or oscillation, as the solid column ofliquid surrounding it tends to dampen an oscillating tendency and toabsorb feedback energy sufliciently to prevent regeneration ofoscillatory impulses.

Where it is preferred to use a back-up member to improve thebreakthrough or exit condition, a preferred arrangement is shown in FIG.2, which illustrates only the workpiece W, a backup member 52, andassociated parts. The back-up member 52, which is made of a lasticsimilar to the plastic from which the bushing 28 is made, is shown asclamped in any suitable mannerfor, example, by means of a C clamp 54tothe workpiece. A recess 56 is cut into the plastic back-up member at itsface which will be clamped against the exit face of the workpiece W, toa depth slightly greater than the thickness (in the direction ofadvance) of the working tip 22, or whatever may be the normallyuninsulated portion of the electrode 20. Extending away from the workand communicating with the recess 56 is a small bleed hole 58. Thisarrangement functions substantialiy as follows: As the electrode 20breaks through the exit surface of the workpiece W, there will be a slugof material, indicated by the reference character 60, whichmay haveeither a generally conical shape (as shown) if the electrode is notintern-ally insulated, or it may have a substantially cylindrical shapeit the interior of the electrode is insulated (in this instance thewords conical and cylindrical are used for illustrative purposes only,because it is clear that the shapes will not be a pure cone or cylinderif the shape of the electrode is other than round). As the breakthroughcontinues, it may be necessary or desirable to prevent having the slug60 hinge on the last remaining point of attachment to the workpiece W insuch a way as to tip and cause short-circuiting against the interioredge of the working tip or against the bore of the electrode if it beuninsulated. The depth of the recess 56 is kept shallow enough toprevent this much cocking of the slug 60. The use of the bleed hole 58assures that the pressure on the rear side 62 of the slug is kept lowerthan that on the front side, so that the pressure differential urges theslug downwardly towards the bottom of the recess 56 without rattling orfloating in an indeterminate manner in the turbulent electrolyte. Whenthe last point of attachment of the slug 60 is eroded away, the slugmoves under the pressure differential so as to close the bleed hole 58.Thus the slug moves out of the way so as to permit a slight additionaladvance of the electrode 20 to clear away any inturned lip at the exitsurface of the work material. A modified form of arrangement to overcomelateral vibration of the electrode is shown in FIG. 3 wherein theelectrode 20 is connected to a header 64 carried on the ram plate 14.The flexible supply conduit 36 from the pump is connected to the header64 by the fitting 38. At the point where the liquid enters the electrode26 from header 64, a plug 90, having a relatively small hole 92therethrough, is fitted into the electrode so as to dampen oscillationin the column of electrolyte. The electrode 20 is shown insulated bothon the inside and outside walls, and the diameter of the working tip ina typical installation is The bore of the tube is .333". The vibrationdampening plug is inserted at the inlet end of the electrode tube and ithas a bore 92 in diameter, and anywhere from to 1" in length. Forsmaller diameter electrodes, smaller damping restrictions will be used.It is believed that the restricted passage 92 introduces a resistiveimpedance to vibratory or oscillatory flow of the electrolyte. It isdesired that the orifice 92 be kept small to do this, but there stillmust be sufficient electrolyte flow to prevent starving of the work gapthrough steaming and gas generation in the electrolyte. The restrictiondescribed with respect to a typical electrode is adequate to furnishenough electrolyte, particularly when the electrode 20 is fed With aclose working gap against the work material. To prevent arcing andsparking a voltage in the order of 7 to 8 volts is used with anelectrolyte solution consisting of pounds of salts in 40 gallons ofwater. While the restrictive orifice 92 provides a sufficient amount ofelectrolyte, it does offer a restriction and friction, if there is anytendency of the liquid column to oscillate in the system at anytroublesome frequency. Under these conditions the adsorption ofoscillation in the narrow passage is too great to permit sustaining theoscillation in the column of electrolyte in the electrode.

in FIG. 4 there is shown another arrangement for preventing vibration ofthe electrode. In this arrangement a very small hole 94 is formed in theelectrode immediately adjacent to its tip 22. After the electrode ismade and before it is insulated, a small hole 94, approximately .060",is drilled just behind the Working tip. When the electrode is dipped inan epoxy type plastic of suflicient fluidity and low enough viscosity tocover the electrode Wit-h a thin film of insulation 24, in the order of.005" in thickness, some of the material naturally is concentratedaround and in the small hole near the tip. In some cases the hole may bepractically closed and remain so after the resin has been cured bybaking at about 350 F. The hole is then enlarged to a diameter ofapproximately .030" using a small drill usually held in a hand tool sothat care can be taken to prevent removing the insulating coating insuch a way as to expose any metal surface, which would cause shortcircuiting in the system. It is probable that the use of this hole,which may be called a weep hole, introduces a discontinuity ordissymetry in the system which breaks up the column of electrolyte inthe electrode 20, thereby preventing the existence of any sustainedoscillation.

While a number of expedients for effecting the objectives of the presentinvention have been shown, it is possible to generalize concerning themto the effect that all of them act upon the column of liquid in thecentral passage or passages of the electrode in such a way as to breakup any tendency of the column of the liquid to oscillate at a regularfrequency which is then translated into lateral vibrations. It is, ofcourse, possible to use various of these expedients in combination witheach other and with electrodes of different sizes and shapes, and it isexpected that this is what will be done. In an attempt to generalizeconcerning what has been done, it should be noted that the varioussystems shown include (1) introducing the electrolyte into the work gaparound the outer surface of the electrode, thereby cushioning the outersurface of the electrode and providing an easy means of regulating anyback pressure in the bore of the electrode by regulating the infeedrate; (2) introducing a resilience into the column of electrolyte in theoutlet from the system; (3) introducing an impedance into the liquidcolumn to retard the formation of oscillation; and (4) introducing adissymetry into the liquid flow pattern.

It is clear from the foregoing that the objectives which have beenclaimed for this invention at the outset of the specification have beenattained.

While preferred embodiments of the new and improved electrolytic cavitysinking apparatus constituting the present invention have been shown anddescribed, it will be apparent that numerous modifications andvariations thereof may be made without departing from the underlyingprinciples of the invention. It is therefore intended, by the followingclaims, to include all such variations and modifications by whichsubstantially the results of this invention may be obtained through theuse of substantially the same or equivalent means.

What is claimed as new and desired to be secured by United StatesLetters Patent is:

1. In electrolytic cavity sinking apparatus having a hollow conductiveelectrode advanced toward and into an electrically conductive andelectrochemically erodable workpiece, means for passing a low voltage,high density direct current between the workpiece and the electrode in asense to make the workpiece anodic and the electrode cathodic, and meansfor pumping an electrolyte from a source to the work gap between theelectrode and the workpiece under high pressure and at a high velocityin the work gap, the combination therewith of an in sulated bushingsurrounding the electrode and sealing against the entry face of theworkpiece to form an electrolyte passage with the exterior of theelectrode, said bushing having a cylindrical bore therein, a tubularpiston-like mount connected to the electrode and slidable in said borein scaling relationship therewith, said tubular mount being connected tothe interior of the electrode to form an electrolyte outlet passage fromthe work gap, and means connecting said electrolyte outlet passage tosaid electrolyte source.

2. In electrolytic cavity sinking apparatus having a hollow conductiveelectrode advanced toward and into an electrically conductive andelectrochemically erodable workpiece, means for passing a low voltage,high density direct current between the workpiece and the electrode in asense to make the workpiece anodic and the electrode cathodic, and meansfor pumping an electrolyte from a source to the work gap between theelectrode and the workpiece under high pressure and at a high velocityin the work gap, the combination therewith of an insulated bushingsurrounding the electrode and sealing against the entry face of theworkpiece to form an electrolyte inlet passage with and around theexterior of the electrode, said bushing having a cylindrical boretherein, a tubular piston-like mount connected to the electrode andslidable in said bore in sealing relationship therewith, the interior ofsaid mount and the interior of the electrode being connected to eachother to provide an electrolyte outlet passage, a restricted electrolyteoutlet from said electrode mount communicating with said outlet passageto impose a back pressure on the electrolyte at the work gap, and meansconnecting the outlet from said outlet passage to said electrolytesupply.

3. In electrolytic cavity sinking apparatus having a hollow conductiveelectrode advanced toward and into an electrically conductive andelectrochemically erodable workpiece, means for passing a low voltage,high density direct current between the workpiece and the electrode in asense to make the workpiece anodic and the electrode cathodic, and meansfor pumping an electrolyte from a supply source to the work gap betweenthe electrode and the workpiece under high pressure and at a highvelocity in the work gap, the combination therewith of an insulatedbushing closely surrounding the electrode and having a surface sealingagainst the entry face of the workpiece in the area surrounding theelectrode to form an inlet passage with and around the exterior of theelectrode, said bushing having a cylindrical bore therein having atransverse area greater than that of said bushing surface sealingagainst the entry face of the workpiece, a tubular piston-like mountconnected to the electrode and slidable in said bore in sealingrelationship therewith, the interior of said mount and the interior ofthe electrode being connected with each other to provide an electrolyteoutlet passage, a restricted electrolyte outlet in said electrode mountcommunicating with said outlet passage to impose a back pressure on theelectrolyte at the work gap, and means connecting said outlet passage:to said electrolyte supply.

4. In electrolytic cavity sinking apparatus having a hollow conductiveelectrode advanced toward and into an electrically conductive andelectrochemically ercdable workpiece, means for passing a low voltage,high density direct current between the workpiece and the electrode in asense to make the workpiece anodic and the electrode cathodic, and meansfor pumping an electrolyte from a supply source to the work gap betweenthe electrode and the workpiece under high pressure and at a highvelocity in the work gap, the combination therewith of an insulatedbushing having a passage therethrough with a transverse configurationcomplemental to that of the external shape of the electrode and closelysurrounding the electrode and from which the electrode projects, thespace between said bushing passage and the external surface of theelectrode forming an electrolyte inlet passage, said bushing having asurface surrounding the passage from which the electrode projects andsealing against the entry face of the workpiece, said bushing having acylindrical bore therein in free communication with said inlet passageand having a transverse area greater than that of said bushing surfacesealing against the entry face of the workpiece, a tubular piston-likemount connected to the electrode slidable in said bore in sealingrelationship therewith and projecting from said bushing in a directionopposite to that of said electrode, the interior of said mount and theinterior of said electrode being connected to each other to provide anelectrolyte outlet passage, a restricted electrolyte outlet in saidelectrode mount where it has projected from said bushing andcommunicating with said outlet passage to impose a back pressure on theelectrolyte in the work gap, and means connecting said electrolyteoutlet passage to said electrolyte source.

5. In electrolytic cavity sinking apparatus having a hollowconductiveelectrode advanced toward and into an electrically conductiveand electrochemically erodable workpiece, means for passing a lowvoltage, high density direct current between the workpiece and theelectrode in a sense to make the workpiece anodic and the electrodecathodic, and means for pumping an electrolyte through the electrode tothe work gap between the electrode and the workpiece under high pressureand at a high velocity in the work gap, the combination therewith of aplug in the inlet end of the electrode having a single flow restrictingpassage therethrough having a diameter about one-fifth that of theelectrode tube, and having a length seven to sixteen times that of thepassage diameter to damp the oscillations of the column of electrolytewithin the hollow electrode.

6. In electrolytic cavity sinking apparatus having a hollow conductiveelectrode advanced toward and into an electrically conductive andelectrochemically erodable workpiece, means for passing a low voltage,high density direct current between the workpiece and the electrode in asense to make the workpiece anodic and the electrode cathodic, and meansfor pumping an electrolyte through the electrode to the work gap betweenthe electrode and the workpiece under high pressure and at a highvelocity in the work gap, the improvement which comprises a bleed holein the side of the electrode against its tip communicating the interiorof the electrode to the atmosphere whereby the formation of oscillationsof the column of electrolyte within the hollow electrode is inhibitedthereby to prevent lateral vibration of the electrode.

7. In electrolytic cavity sinking apparatus having a hollow conductiveelectrode advanced toward and through an electrically conductive andelectrochemically erodable workpiece, means for passing a low voltage,high density direct current between the workpiece and the electrode in asense to make the workpiece anodic and the electrode cathodic, and meansfor pumping an electrolyte from a source to the work gap between theelectrode and the workpiece under high pressure and at a high velocityin the work gap, the combination therewith of means surrounding theelectrode and sealing against the entry face of the workpiece to form anelectrolyte passage between the exterior of the electrode and said lastnamed means, pressure operated means connected to said passage to urgesaid sealing means against the entry face of the workpiece, meansconnecting said electrolyte source to said passage, and a backup memberat the exit surface of the workpiece having a shallow cavity therein toreceive without appreciable cocking the slug of material from theworkpiece cavity, said backup member having a restrictive outlet passagefrom said cavity and opening to the atmosphere.

References Cited by the Examiner UNITED STATES PATENTS 2,308,860 1/1943Clark -3. 255-1 2,818,490 12/1957 Dixon et a1 219-69 2,909,641

FOREIGN PATENTS 7/1929 Great Britain. 5/1962 Germany.

JOHN H. MACK, Primary Examiner.

10/ 1959 Kucyn 21969 10 W. VAN SISE, Assistant Examiner.

1. IN ELECTROLYTIC CAVITY SINKING APPARATUS HAVING A HOLLOW CONDUCTIVEELECTRODE ADVANCED TOWARD AND INTO AN ELECTRICALLY CONDUCTIVE ANDELECTROCHEMICALLY ERODABLE WORKPIECE, MEANS FOR PASSING A LOW VOLTAGE,HIGH DENSITY DIRECT CURRENT BETWEEN THE WORKPIECE AND THE ELECTRODE IN ASENSE TO MAKE THE WORKPIECE ANODIC AND THE ELECTRODE CATHODIC, AND MEANSFOR PUMPING AN ELECTROLYTE FROM A SOURCE TO THE WORK GAP BETWEEN THEELECTRODE AND THE WORKPIECE UNDER HIGH PRESSURE AND AT A HIGH VELOCITYIN THE WORK GAP, THE COMBINATION THEREWITH OF AN INSULATED BUSHINGSURROUNDING THE ELECTRODE AND SEALING AGAINST THE ENTRY FACE OF THEWORKPIECE TO FORM AN ELECTROLYTE PASSAGE WITH THE EXTERIOR OF THEELECTRODE, SAID BUSHING HAVING A CYLINDRICAL BORE THEREIN, A TUBULARPISTON-LIKE MOUNT CONNECTED TO THE ELECTRODE AND SLIDABLE IN SAID BOREIN SEALING RELATIONSHIP THEREWITH, SAID TUBULAR MOUNT BEING CONNECTED TOTHE INTERIOR OF THE ELECTRODE TO FROM AN ELECTROLYTE OUTLET PASSAGE FROMTHE WORK GAP, AND MEANS CONNECTING SAID ELECTROLYTE OUTLET PASSAGE TOSAID ELECTROLYTE SOURCE.